Road vehicles -- Measurement techniques in impact tests -- Optical instrumentation

This document defines performance criteria for an optical data channel used in impact tests on road vehicles, when numerical time and space data are taken from images to analyse impact test results. The objective of this document is to facilitate comparison between results obtained by different laboratories by specifying minimum quality criteria. Annexes A, B, C and D present a method of measuring several indices like quality parameters of sub processes of the optical data channel, using a calibration target, reference distances and analysis systems.

Véhicules routiers -- Techniques de mesure lors des essais de chocs -- Instrumentation optique

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Publication Date
24-Jul-2018
Current Stage
6060 - International Standard published
Start Date
30-Jun-2018
Completion Date
25-Jul-2018
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INTERNATIONAL ISO
STANDARD 8721
Third edition
2018-08
Road vehicles — Measurement
techniques in impact tests — Optical
instrumentation
Véhicules routiers — Techniques de mesure lors des essais de chocs —
Instrumentation optique
Reference number
ISO 8721:2018(E)
ISO 2018
---------------------- Page: 1 ----------------------
ISO 8721:2018(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2018

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

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Published in Switzerland
ii © ISO 2018 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 8721:2018(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Symbols and abbreviated terms ........................................................................................................................................................... 5

5 Performance .............................................................................................................................................................................................................. 7

5.1 General requirements ....................................................................................................................................................................... 7

5.2 Reference distance ............................................................................................................................................................................... 7

5.3 Time base system.................................................................................................................................................................................. 7

5.4 Performance of the optical data channel .......................................................................................................................... 7

5.4.1 General...................................................................................................................................................................................... 7

5.4.2 Performance indices ..................................................................................................................................................... 7

5.4.3 2D performance value ................................................................................................................................................. 8

5.4.4 3D performance value ................................................................................................................................................. 8

5.4.5 Performance value of the optical data channel ..................................................................................... 9

5.5 Accuracy of the optical data channel ................................................................................................................................... 9

5.5.1 Accuracy indices ............................................................................................................................................................... 9

5.5.2 Length measurement error and accuracy value of a reference distance .....................10

5.5.3 Length measurement error and accuracy value of the optical data channel ............10

5.6 Types of procedure ...........................................................................................................................................................................11

5.6.1 General...................................................................................................................................................................................11

5.6.2 Type of procedure — Online ..............................................................................................................................11

5.6.3 Type of Procedure — Offline ..............................................................................................................................11

5.7 Conformity statement ....................................................................................................................................................................12

5.8 Derived quantities .............................................................................................................................................................................12

5.9 User-defined variables ..................................................................................................................................................................12

6 Documentation ....................................................................................................................................................................................................13

Annex A (normative) Index determination methods .......................................................................................................................14

Annex B (informative) Measurement methods ......................................................................................................................................40

Annex C (informative) Clarification of parameters ............................................................................................................................41

Annex D (informative) Dependences between the indices and the variables ........................................................43

Bibliography .............................................................................................................................................................................................................................44

© ISO 2018 – All rights reserved iii
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ISO 8721:2018(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following

URL: www .iso .org/iso/foreword .html

This document was prepared by Technical Committee ISO/TC 22, Road vehicles, Subcommittee SC 36,

Safety and impact testing.

This third edition cancels and replaces the second edition (ISO 8721:2010), of which it constitutes a

minor revision with editorial changes.
iv © ISO 2018 – All rights reserved
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INTERNATIONAL STANDARD ISO 8721:2018(E)
Road vehicles — Measurement techniques in impact tests
— Optical instrumentation
1 Scope

This document defines performance criteria for an optical data channel used in impact tests on road

vehicles, when numerical time and space data are taken from images to analyse impact test results.

The objective of this document is to facilitate comparison between results obtained by different

laboratories by specifying minimum quality criteria.

Annexes A, B, C and D present a method of measuring several indices like quality parameters of sub

processes of the optical data channel, using a calibration target, reference distances and analysis

systems.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 6487, Road vehicles — Measurement techniques in impact tests — Instrumentation

3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— IEC Electropedia: available at http: //www .electropedia .org/
— ISO Online browsing platform: available at https: //www .iso .org/obp
3.1
analysis system

system to measure and collect the coordinates of target points in image space as a function of time

Note 1 to entry: The calculation results of the analysis system are 3D coordinates in object space, whereas in the

case of 2D analysis, the depth of the target points is known and considered.
3.2
cell size
distance of neighbouring pixels on the sensor of an image recording device

Note 1 to entry: If there are different distance values in the two main directions of the image, the cell size is the

maximum of these values.
3.3
control point

point that was determined with a higher accuracy and is further accepted as an error-free point

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ISO 8721:2018(E)
3.4
frame rate

frequency of renewal of information for a given point, expressed in renewals per second, or in images

per second if all points of the image are renewed simultaneously
3.5
image recording device
system composed of a camera/lens unit together with a recording system
3.6
location accuracy
loc
desired accuracy of the object or target being measured
3.7
optical data channel

system composed of one or more image recording devices and a system for analysing the images,

including any analysis procedure and data correction that validate and modify the content of the data

3.8
reference distance
known distance between a validation target pair
3.9
synchronism device

device to identify the synchronism effect in two or more corresponding image recording devices

3.10
time base system

device allowing determination of the time interval elapses between any two recorded events for each

image recording device
3.11
time origin identification device

device to identify the instant chosen as the time origin, usually the contact between the test objects

3.12
validation target pair

pair of targets placed in the field of view so that the distance separating them remains constant

Note 1 to entry: Both of them are visible during the impact test.
3.13
accuracy value

value that represents the relative overall accuracy of any point measurement within the optical data

channel when the performance value is satisfied
3.14
accuracy value limit
avl

user-defined limit for the accuracy value that represents the relative overall accuracy of any point

measurement within the optical data channel when the performance value is satisfied

3.15
camera position calculation index
cpc

index that gives the possibility to evaluate whether the accuracy of the optical data channel determined

from one time step is representative for the entire sequence
2 © ISO 2018 – All rights reserved
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ISO 8721:2018(E)
3.16
camera set-up index

index that makes it possible to evaluate whether the set-up of the camera with respect to the movement

plane permits a reliable analysis
Note 1 to entry: Only for 2D film analysis.
3.17
control point distribution index
cpd

index that makes it possible to evaluate whether the distribution of the control points in the image

permits a reliable orientation of the used images
3.18
distortion index

index that makes it possible to evaluate whether the interior orientation parameters of the used camera

are still valid
3.19
focal length index

index that makes it possible to evaluate whether the focal length of the used image recording device is

still valid
3.20
index value

value that is determined by its index calculation equation (one value for each index)

Note 1 to entry: For the different index calculation equations see Annex A.

Note 2 to entry: The index value is the result of the index determination and is a floating point number.

3.21
index condition
condition of the check of the index

Note 1 to entry: The index condition can be true (value 1) or false (value 0). A true or false measure indicating if

an index is within its required limits.”
3.22
intersection index

index that makes it possible to evaluate the intersection geometry of the rays from the image recording

devices to the object points
Note 1 to entry: Only for 3D film analysis.
3.23
length measurement error

value that represents the absolute overall accuracy of any point measurement within the optical data

channel when the performance value is satisfied
3.24
motion blur index

index that allows one to evaluate whether the exposure time used in the test is small enough with

respect to the appropriate object movement, in order to ensure a reliable point identification and point

measurement in the images
© ISO 2018 – All rights reserved 3
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ISO 8721:2018(E)
3.25
performance value

numerical value indicating suitable general conditions for the estimation of the accuracy of the optical

data channel

Note 1 to entry: It is derived from all indices which describe the performance of the optical data channel.

3.26
plane scale index

index used to rate the appropriateness of evaluating the scale constellation in the movement planes

(scale in each movement plane)
Note 1 to entry: Only for 2D film analysis.
3.27
point motion index

index used to rate the appropriateness of the frame rate in regards to test requirements

3.28
scale index

index used to rate the appropriateness of the scale constellation to control the system scale (presence

of independent reference distances)
3.29
synchronism index

index used to rate the appropriateness of the synchronization method for the data analysis in regards

to the test requirements
Note 1 to entry: Only for 3D film analysis.
3.30
target detection index

index used to rate the appropriateness of the measuring accuracy of the image coordinates in regards

to test requirements
3.31
target size index

index used to rate the appropriateness of the target size for a reliable point identification and point

measurement in the images
3.32
time base index

index used to rate the appropriateness of the used time base system in regards to the test requirements

3.33
time origin identification index
toi

index used to rate the appropriateness of the used time origin identification device in regards to the

test requirements
4 © ISO 2018 – All rights reserved
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ISO 8721:2018(E)
4 Symbols and abbreviated terms
Symbol Definition
A control point formed area
A image area
a accuracy value of the optical data channel
a allowed location accuracy in depth
alaid
a current location accuracy (distortion)
clad
a current location accuracy (focal length)
claf
a current location accuracy (intersection)
clai
a current location accuracy in depth
claid
a current location accuracy (target)
clat
a current location accuracy (time base)
clatb
a current location accuracy (time origin identification)
clatoi
a distortion accuracy
a focal length accuracy
a frame rate accuracy
a location accuracy
loc
a accuracy value of the reference distance, r
refdist,r
a target detection accuracy
d object distance
e exposure time
f focal length
f frame rate
i camera position calculation index
cpc
i control point distribution index
cpd
i camera set-up index
i distortion index
i focal length index
i intersection index
i motion blur index
i point motion index
i plane scale index
i scale index
i synchronism index
i time base index
i target detection index
i time origin identification index
toi
i target size index
ΔL length measurement error of the optical data channel
Δl length measurement error of reference distance, r
l asynchronism effect in viewing direction
aed
l asynchronism effect perpendicular to the viewing direction
aep
l allowed point motion between two sequenced images in object space
apm
l calibrated length of reference distance, r
c,r
l camera base
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ISO 8721:2018(E)
Symbol Definition
l current motion blur value
cmbv
l current point motion between two sequenced images in object space
cpm
l cell size
l current target diameter
ctd
l distance camera base to object
dco
l fix point distance
fpd
l image height
l image width
l (t) measured length of reference distance, r, as a function of time
m,r
l maximum displacement in image space
mdi
l maximum displacement in object space
mdo
l required target diameter
rtd
l theoretical target diameter
ttd
p 3D performance value of the optical data channel
p control point area
cpa
p control point distribution
cpd
p distance to plane of motion i
dtp,i
p number of planes of motion
p reference distance
rd,r
p reference distance in direction i
rd,i
p reference distance in plane of motion i
rdp,i
p scale information in all planes of motion
siap
p scale information in plane of motion i
sip,i
p synchronism index in viewing direction
syd
p synchronism index perpendicular to the viewing direction
syp
p target in image section i
t,i
p type of camera set-up
tpc
p type of position determination
tpd
Q performance value of the optical data channel
q 2D performance value of the image recording device i
r allowed accuracy relation
aar
r accuracy value limit
avl
r current accuracy relation
car
t beginning of the analysed time interval
t user-defined time within the analysed time interval
t current asynchronism
t difference between t -image and -signal
dtz 0
t end of the analysed time interval
t time interval
int
t time drift
t total time drift
ttd
v velocity
6 © ISO 2018 – All rights reserved
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ISO 8721:2018(E)
5 Performance
5.1 General requirements

The performance of the optical data channel shall be evaluated initially to establish performance levels.

This evaluation shall be repeated whenever the system is modified to an extent which could cause a

change in accuracy. This shall be done with an offline procedure.

It is also possible to measure the performance of the optical data channel during an impact test. This is

called the online procedure.

The performance of the optical data channel shall be estimated using 2D performance values, or 3D

performance values, or both. These values consist of different performance indices depending on the

test constellation. To verify the estimated performance values, an accuracy value shall be determined

using two or more reference distances.

If a film analysis is carried out using the image sequences of on-board cameras, the used equipment

(camera and lens) shall correspond to the expected shock.
Further details are described in the annexes.

The determination methods for several indices are described in Annex A and shall be used for the index

calculation.

Details for the two measurement methods (online and offline procedure) are listed in Annex B.

A complete list of all used parameters with short text, long text and the type is in Annex C.

The matrix structure in Annex D presents the dependencies between the indices and the parameters.

5.2 Reference distance

The reference distances shall be determined 10 times more precisely than the desired location accuracy.

The determination of the reference distances shall be done before the test.

The reference distances shall be located on approximately perpendicular (90 ± 10)° lines (see A.3.2).

For 3D analysis, all three directions in space shall be covered. See Annex A for additional information.

5.3 Time base system

The time base shall be determined 10 times more precisely than the desired time accuracy.

5.4 Performance of the optical data channel
5.4.1 General

The performance of the optical data channel consists of different indices (see Table 1). The determination

depends on the application (2D or 3D).
5.4.2 Performance indices

Each index value shall be at least 0,5. If this minimum requirement is not fulfilled for every index, then

the impact test does not conform to this document. The index condition of a certain index is 0 if the

requirements for this index (see Annex A) are not fulfilled; otherwise the index condition is 1.

© ISO 2018 – All rights reserved 7
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ISO 8721:2018(E)
Table 1 — Performance indices
Number per optical
Index 2D 3D Comment
data channel
a a
Focal length index one per image recording device in a suitable image
a a
Distortion index one per image recording device in a suitable image
a a

Target detection index one per image recording device worst target used in the analysis

a a

Target size index one per image recording device worst target used in the analysis

a a
Motion blur index one per image recording device at maximum object speed
a a
Point motion index one per image recording device at maximum object speed
Control point distribution
a a
one per image recording device in a suitable image
index
a a
Time base index one per image recording device —
Time origin identification
a a
one per image recording device —
index
a b
Camera set-up index one per image recording device —
a b
Plane scale index one per image recording device —
best pair of image recording
b a
Intersection index one
devices
worst pair of image recording
b a
Synchronism index one
devices
Index value is used for the performance value.
Index value is not used for the performance value.
5.4.3 2D performance value

The performance value for every image recording device is estimated by all 2D related index conditions

(see Table 1). The 2D performance value, q , is the ratio of the achieved sum to the possible sum of index

conditions with respect to the test requirements, and is calculated as shown in Formula (1):

∑ ji
j=1
q = (1)
where
i is the image recording device number;
j is the 2D performance index number;

x is the index condition of the 2D performance index, j, of the image recording device, i;

n is the number of 2D performance indices (2D film analysis: n = 11; 3D film analysis: n = 9).

5.4.4 3D performance value

The 3D performance value of the optical data channel, p, is calculated as shown in Formula (2):

py= (2)
k=1
where
8 © ISO 2018 – All rights reserved
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ISO 8721:2018(E)
k is the 3D performance index number;

y is the index condition of the 3D performance index, k, of the optical data channel;

m is the number of 3D performance indices (m = 2).
5.4.5 Performance value of the optical data channel

For 2D analysis, the performance value of the optical data channel, Q, is identical to the 2D performance

value, q , as shown in Formula (3):
Qq= (3)

For 3D analysis with only one image recording device, the intersection index and the synchronism

index are not defined. In this case, the performance value of the optical data channel, Q, is equal to the

2D performance value, q .

For 3D analysis, the performance value of the optical data channel, Q, is the ratio of the achieved sum to

the possible sum of all index conditions, calculated according to Formula (4):
u u
   
   
nq× +×()pu × qp+
∑∑i i
   
i==11i
   
Q= = (4)
nu× +×mu n++m
() ()
where
i is the image recording device number;
q is the 2D performance value of the image recording device, i;
u is the number of image recording devices;

n is the number of 2D performance indices (2D film analysis: n = 11; 3D film analysis: n = 9);

m is the number of 3D performance indices (m = 2);
p is the 3D performance value of the optical data channel.
5.5 Accuracy of the optical data channel
5.5.1 Accuracy indices
The accuracy indices are shown in Table 2.
Table 2 — Accuracy indices
Number per optical data
Index Comment
channel
Camera position calculation
one per image recording device —
index
Scale index one indispensable index
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ISO 8721:2018(E)
5.5.2 Length measurement error and accuracy value of a reference distance

The length measurement error and accuracy value of a reference distance are defined as follows:

— the length measurement error, Δl , of the reference distance, r, is the maximum difference between

the measured length, l (t), and the calibrated length, l , within the analysed time interval;

m,r c,r

— the accuracy value, a , of the reference distance, r, is the maximum relative difference between

refdist,r

the measured length, l (t), and the calibrated length, l , within the analysed time interval.

m,r c,r

All used image recording devices shall be used for the calculation of the reference distances.

If the index condition of the camera position calculation index, i , of all used image recording devices

cpc

is fulfilled, the length measurement error, Δl , can be determined at a single time step within the

analysed time interval. If the index condition of only one image recording device is not fulfilled, the

length measurement error, Δl , shall be calculated for every time step within the analysed time interval.

The accuracy value, a , of the reference distance, r, is the ratio between the length measurement

refdist,r
error, Δl , and the calibrated length, l .
r c,r

If every i ≥ 1, then the length measurement error, Δl , is calculated according to Formula (5):

cpc,i r
Δll=−()tl (5)
rrm,,cc r
where

i is the index value of the camera position calculation index of the image recording device, i;

cpc,i
i is the image recording device number;
r is the reference distance number;
l (t) is the measured length of reference distance, r, as a function of time;
m,r
l is the calibrated length of reference distance, r.
c,r

If any i < 1, then the length measurement error, Δl , is calculated according to Formula (6):

cpc,i r
Δll=−max(tl) (6)
rrmc,,r
where
t is the beginning of the analysed time interval;
t is the end of the analysed time interval;
t is a user-defined time within the analysed time interval.
The accuracy value, a , is calculated according to Formula (7):
refdist,r
a = (7)
refdist,r
c,r
5.5.3 Length measurement error and accuracy value of the optical data channel

— The length measurement error of the optical data channel, ΔL, is the maximum of the length

measurement errors, Δl , of all reference distances, r.

— The accuracy value of the optical data channel, a, is the maximum of the accuracy values, a , of

refdist,r
all reference distances, r
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ISO 8721:2018(E)
ΔΔLl=max (8)
aa=max (9)
refdist,r
5.6 Types of procedure
5.6.1 General

Conformity with this document can be verified by different types of procedure, depending on the

desired complexity. The different types of procedure are shown in Table 3.
Table 3 — Types of procedure
Before the real impact test During the real impact test
Type of
procedure
Performance value Accuracy value Performance value Accuracy value
ΔL ≤ a
loc
Online — — Q ≥ 0,7
a ≤ r
avl
Synchronism index
ΔL ≤ a ΔL ≤ a
loc loc
Offline Q ≥ 0,8 i ≥ 1
a ≤ r a ≤ r
avl avl
(only 3D analysis
...

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